Saturday, April 9, 2011

Alice and Bob or Proxies and the Rabbis

In the famed Jargon Dictionary, there is an entry for Alice and Bob:
Alice and Bob: n.
The archetypal individuals used as examples in discussions of cryptographic protocols. Originally, theorists would say something like: “A communicates with someone who claims to be B, So to be sure, A tests that B knows a secret number K. So A sends to B a random number X. B then forms Y by encrypting X under key K and sends Y back to A” Because this sort of thing is quite hard to follow, theorists stopped using the unadorned letters A and B to represent the main players and started calling them Alice and Bob. So now we say “Alice communicates with someone claiming to be Bob, and to be sure, Alice tests that Bob knows a secret number K. Alice sends to Bob a random number X. Bob then forms Y by encrypting X under key K and sends Y back to Alice”. A whole mythology rapidly grew up around the metasyntactic names; see
In Bruce Schneier's definitive introductory text Applied Cryptography (2nd ed., 1996, John Wiley & Sons, ISBN 0-471-11709-9) he introduced a table of dramatis personae headed by Alice and Bob. Others include Carol (a participant in three- and four-party protocols), Dave (a participant in four-party protocols), Eve (an eavesdropper), Mallory (a malicious active attacker), Trent (a trusted arbitrator), Walter (a warden), Peggy (a prover) and Victor (a verifier). These names for roles are either already standard or, given the wide popularity of the book, may be expected to quickly become so.

How did the Rabbis handle the same problem?

They used the names of the twelve tribes of Israel, Reuven Shimon, Levi, etc. as proxies for A, B, and C. For example, Ibn Ezra comments, while explaining the differences in the two presentations of the Ten Commandments,
And behold, I will give you an allegory. A person says to you, "Write to my friend, and this is what you should write: I, Ploni love you forever." And he writes Ploni, פלני, without the vav {which is acceptable in Hebrew}, and אהבך, 'I love you', also without the vav. And לעלם, "forever" also chaser. {In each case you can have a cholam chaser as opposed to a cholam malei.} And then Reuven comes and asks me, "why did you write them deficiently {chaser}? And meanwhile, I only have need to write that which he said to me, and I have no desire that they be either malei or chaser. Perhaps Levi will come and inform me how I shall write. I do not wish to go on at length. Rather, the intelligent person will understand. (Ibn Ezra on Ex. 20:1)
Here, Reuven and Levi are merely generic names, much like (the more polically correct) Alice and Bob, and distinguished from ploni or ploni almoni (literally, ploni anonymous) which is used to refer to a person whose name we do not know, similar to the use of John Doe in Western Law.

Similarly, The law of the pursuer (rodef): In the Mishnah Sanhedrin 8:7 it is stated that,
if a person (Reuven) sees someone (Shimon) running after a person in order to kill or rape that person, then Reuven may kill Shimon in order to prevent the crime.
This law is explained at length by the Talmud and codes (Babylonian Talmud Sanhedrin 73a) and by Maimonides (Laws of Murder 1:6). One may kill a rodef only if one sees him pursuing another person with the evident intention to kill that person, and killing the rodef is the only way to save this person.[1]

As Rabbi Julian Sinclair wrote in The Jewish Chronicle, (May 27, 2010):
"While English uses phrases like John Doe or A N Other to refer to an unnamed or unspecified person, in Hebrew we say ploni almoni or just ploni. In discussing hypothetical scenarios, the Talmud uses ploni to refer to a witness, a bridegroom, or whatever the case maybe.
Ploni Almoni appears in Ruth, as the late Elimelech's relative who should marry Ruth but refuses. Rashi explains ploni to mean covered and hidden, deriving from a word looked at recently in this column - peleh (Ruth 4:1). Almoni comes from ilem, meaning silenced or muted. Ploni Almoni turned away from the historic act of embracing Ruth into the Jewish people. Therefore it is only fitting that his real name be blocked out.
In I Samuel David, in flight from Saul, does not wish to disclose his destination to the priest Ahimelech, so he says, "I have directed my men to the place of ploni almoni" (21:3).
In the Bible, ploni almoni is used when one wants to conceal information deliberately. In the Talmud and in modern Hebrew, ploni almoni is simply a useful expression to refer to no one in particular."

[1] Monique Susskind Goldberg, "Is it Permitted to Kill Another Jew?" The Schechter Institutes (, accessed January 23, 2011), and based on Arthus Waskow, "Jewish Law on the Killing of Yitzhak Rabin," November 14, 2005, (, accessed January 23, 2011).

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